1. Field of the Invention
Embodiments of the invention relate to a gamma reference voltage generating circuit and a display device including the same.
2. Discussion of the Related Art
Various flat panel displays (FPDs), which may replace cathode ray tubes (CRTs) with disadvantageous weight and volume, have been developed. Examples of the flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an organic light emitting display. Most of the flat panel displays have been put to practical use and have been selling.
In the flat panel displays, the liquid crystal display represents a gray scale by controlling a light transmittance of a liquid crystal layer included in a display panel depending on a magnitude of a driving voltage applied to the display panel. Further, the organic light emitting display represents a gray scale by controlling an amount of current flowing in an organic light emitting diode depending on a magnitude of a driving voltage applied to a display panel.
In general, a gray scale means that an amount of light, which a person perceives through his or her eyes, is divided in stages. According to Weber's law, human eyes respond nonlinearly to brightness of light. Because of this, when a person perceives changes in brightness of light through his/her eye and linearly measures the changes in the brightness of light within a limited bit depth (for example, k-bit) per channel, he/she does not smoothly feel the brightness of light and perceives the intermittent change in the brightness of light. Thus, the brightness of light needs to be nonlinearly decoded, so as to achieve the optimum image quality within the limited bit depth. For this, a process for matching driving characteristics of the display panel and characteristics perceived through human eyes is required. The matching process is called a gamma correction. A gamma correction method generally includes setting a plurality of gamma reference voltages based on the driving characteristics of the display panel, dividing each of the set gamma reference voltages, and compensating for a gamma value of each of input digital video data.
The gamma reference voltages are generated by using voltage followers, which are respectively connected to reference voltages generated by a plurality of digital-to-analog converters (DACs), or by connecting the voltage followers to voltages divided by a resistor string. The voltage followers stabilize the reference voltages and output final gamma reference voltages. The voltage followers implemented as buffers induce an overcurrent due to a difference between slew rates of chips at the moment that the gamma reference voltages are changed.
For example, when a slew rate of a first voltage follower is less than a slew rate of a second voltage follower, a change rate of the voltage in a process for changing a first gamma reference voltage gamma1 and a second gamma reference voltage gamma2 is shown in FIG. 1. Namely, as shown in FIG. 1, because a change rate of the first gamma reference voltage gamma1 is less than a change rate of the second gamma reference voltage gamma2 in a voltage change period Tt, there is a dramatic change in a difference between the first gamma reference voltage gamma1 and the second gamma reference voltage gamma2 at a resistance node between the first and second gamma reference voltages gamma1 and gamma2 Thus, overcurrent is instantaneously generated at a node of an output resistance.